<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>定位练习</title>
    <style>
        body {
            margin: 100px;
            height: 800px;
            border: 1px solid red;
        }
        header {
            width: 100px;
            height: 100px;
            background-color: seagreen;
            position: fixed;
            top: 100px;
            left: 0;
            /* 改变图层的显示顺序 */
            /* z-index: 1; */
        }
        /* aside {
            width: 100px;
            height: 100px;
            background-color: sienna;
            position: absolute;
            left: 20px;
            top: 20px;
            z-index: 2;
        } */
    </style>
</head>
<body>
    <header>header</header>
    <!-- <aside>aside</aside> -->
    111111
</body>
</html>